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Prop stuff, over revving

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Rondo64

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Hello all. I have a question to the forum. I have a 2013 20sl with a 2012 Yamaha F70. Nice boat and engine. 25" pontoons (2), with the short center fuel pod. Engine is on lowest hole. I talked with Benny and they said since I have the upgraded toons my max hp is 90 rather than the sticker 70hp. I asked for prop recommendations from them and they suggested the 14x11 dual thrust elephant ear-looking prop. So I bought it. Looks like a rooster with socks, but it works. I jump up to almost max speed in a second with almost no throttle. Like 1/4. But, if I push it much further, it'll peg out at 7500 rpm. No, I don't run it like that, but it leaps to 7500. It feels like I'm ventilating big time, but I am trimmed all the way in. The vent plate is about an inch below the fuel pod bottom. So, 3 possibilities in my brain. 1, the pitch isn't right. But, I'd have to add 2 or 3 inches to even get close to 6000. 2, because of upgraded toon and extra flotation, engine is too high (boat floats higher so transom is too high) , or 3, the fuel pod is cramming soft water into prop. None of those are mutually exclusive. I am not inclined to do much with any of the variables until I gleen max wisdom from you guys.

Thanks
 
Based solely on a year of reading posts and stories: #2 is the winner - engine is too high. That should be easy to determine, noting that the engine will rise as you accelerate - need someone to drive on a calm smooth day while you peek at the water level.
 

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That's my thought too. With the bigger toons the pod rides higher. Hard to check and hard to do anything about it!
 
Wow, that's a pretty serious problem. Without some speed versus RPM notes, sounds like the prop is unloading bigtime. Check not only the mounting hole, but if the motor bracket is sitting on the transom (maybe the holes were not drilled where they need to be). Even with our 23" tubes, our motor works best full down (and the motor bracket is on the transom).

If you are stuck with the config, more prop blade area (and maybe more pitch) should help. A 4-blade may be what you need (like the Nemesis we run, turning point makes them too). Before you go just trial and error, know the gear ratio of you motor. Sounds like the current setup is so bad that it's not useful as a guide for what you really need....

As an after-thought, are you comfortable that the RMP you are seeing is correct?
 
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Thanks for the feedback. I talked with Bennington and they confirmed that the toons raise the stern. Duh. But, with the upgraded toons they didn't compensate by adjusting the transom down. Engine is mounted on lowest setting but bracket is maybe 3/4 inch off the transom. I suppose I could drill new holes or put a spacer to drop it a tad, but worry that I get backwash into cowl when coming off the throttle. Maybe a 4 blade would grip better but I hate to keep throwing props at it. If I could extend the leg then I could tune the height. Or sell it and buy another. Or paddle really fast.
 
Rondo64 said:
Thanks for the feedback. I talked with Bennington and they confirmed that the toons raise the stern. Duh. But, with the upgraded toons they didn't compensate by adjusting the transom down. Engine is mounted on lowest setting but bracket is maybe 3/4 inch off the transom. I suppose I could drill new holes or put a spacer to drop it a tad, but worry that I get backwash into cowl when coming off the throttle. Maybe a 4 blade would grip better but I hate to keep throwing props at it. If I could extend the leg then I could tune the height. Or sell it and buy another. Or paddle really fast.
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Engine Mounting is generally a Dealer Issue, not Bennington. My transom has 2 giant holes in the bottom to let water in and out - so drilling new holes if needed would not be an issue. Sounds like you have an unusual configuration - pictures might help.
 
Could be the prop/engine height is too high and venting - you could try a lower diameter prop and see if that helps, even an inch can make all the difference. Or try a different prop design, some are stern-lifting and can bring the prop tips closer to the surface and and introduce air. Props with more rake tend to be more bow-lifting and can be lower in the water.

Sometimes there are things that are entraining the water with air forward of your prop. Could be an attachment like a transducer or thru-hull or crud.

Or another possibility is there is something off with the prop itself - if it's not seated correctly then exhaust gas can escape and cause venting, or some props have vent plugs that intentionally introduce exhaust gas for better hole shot but might make a bad situation worse.

Bottom line, there could be many reasons why this is happening! Going to an F90 25" shaft would run $10k just for the engine but might be able to offset a large part of it by selling the F70.
 
Thanks for all the help. To your point, the prop Benny recommended is the dual thrust, which shunts exhaust forward when in reverse to give the prop clean water in reverse. Thanks for the lead on whether exhaust is aerating the water. I'll study the prop function to see if that theory bears fruit.
 
I didn't catch the DT model the first time around but that would certainly be high on my list of suspects. Low rake and big diameter made for pushing heavy boats with small engines, but with the height issue those two design features are almost certainly (in my mind) working against you.

I know you don't want to start throwing money at the prop dart board but that would be my first stop. Hard to find a smaller diameter prop without a ton of pitch but just a standard Yamaha aluminum 6E5-45949-00-00 (13 5/8 x 13) might be cheap enough to try out.
 
That's where my head is. A 14x11 or a 13.5x12 too. I have a 14x13 with (too much) cup and rpm tops out at 5000. I had my prop guy put cup because the 14x13 would turn 6000 with no increase in velocity from 5000 to 6000...bad slippage. Figure too much pitch at 13, so opted for 14x11. But, that dual thrust is the wild card. I'd really like to throw a 90 hp on it but would like to squeeze that 70 turnip for all it has.

Again, thanks.
 
Occam's Razor is a parable of sorts that says the best solution is generally the simplest, most obvious. Case in point, in my situation the tach was wrong. With a hand held tach as reference, boat tach was 1000 rpm too high. The pulse setting was wrong. Now, registering 6200 WOT with a 14x11. Almost there, but I've got it by the short hairs now.

Thanks to all for being excellent thinking partners.
 
Good to read you did check the tach with a "second opinion" and now have the right setting. Doubt there is much to gain with a different prop, especially considering what corrected results would be with either the 13p before or after modification. Maybe there is no need to tinker with getting the motor bracket down on the transom?

Have you had a chance to get a GPS speed yet?
 
Hi. Not going to tinker much more. May do an inch increase on prob, but ugh. Gps speed went from about 17mph with the 14x13 to 21mph with 14x11.
 
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